April 4-6

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Author: Felix I. Parra
Requested Type: Poster
Submitted: 2022-03-04 08:20:42

Co-authors: V. d'Herbemont, I. Calvo, J.L. Velasco

Contact Info:
Princeton Plasma Physics Laboratory
100 Stellarator Road
Princeton, New Jerses   08540

Abstract Text:
A new set of orbit-averaged equations for low-collisionality neoclassical fluxes in large aspect ratio stellarators is derived [1]. The equations are self-consistently local while retaining finite orbit width effects via the use of the second adiabatic invariant as a coordinate. These new equations have been implemented in the orbit-averaged neoclassical code KNOSOS [2]. The new equations are used to study the 1/nu regime and the regimes that exist at lower collisionality. For generic large aspect ratio stellarators, as the collision frequency decreases, the 1/nu regime transitions directly into the nu regime, without passing through a sqrt(nu) regime. An explicit formula for the particle and heat fluxes in the nu regime is obtained. The formula includes the effect of particles that transition between different types of wells. While these transitions make the particle motion stochastic, they only produce stochastic diffusion independent of the value of the collision frequency in velocity space. The diffusion in real space remains proportional to the collision frequency, as it should in the nu regime. The sqrt(nu) regime is only recovered in large aspect ratio stellarators close to omnigeneity, and it only exists in an interval of collisionality that depends on the deviation from omnigeneity.

[1] V. d'Herbemont et al., submitted to J. Plasma Phys., arXiv:2202.01318 (2022).
[2] J.L. Velasco et al., J. Comput. Phys. 418, 109512 (2020); J.L. Velasco et al., Nucl. Fusion 61, 116013 (2021).